header {
    /* display: block; */
    margin: 0 auto;
    width: 500px;
    text-align: center;
}

header h1 {
    font-family: Arial;
    font-size: 40px;
    font-weight: bold;
}

#newgamebutton {
    /* 为了设置各方向的padding和margin，将其变成块元素 */
    display: inline-block;
    margin: 5px 20px;
    width: 100px;
    color: white;
    background-color: #8f7a66;
    text-decoration: none;
    padding: 10px;
    border-radius: 10px;
    font-family: Arial;
}

#newgamebutton:hover {
    background-color: #9f8b77;
}

#undobutton {
    display: inline-block;
    margin: 5px 20px;

    width: 100px;
    padding: 10px 10px;
    background-color: #8f7a66;

    font-family: Arial;
    color: white;

    border-radius: 10px;

    text-decoration: none;
}

#undobutton:hover {
    background-color: #9f8b77;
}

header p {
    font-family: Arial;
    font-size: 25px;
    margin: 20px auto;
}

#grid_container {
    width: 460px;
    height: 460px;
    padding: 20px;
    margin: 30px auto;
    border-radius: 10px;
    background-color: #bbada0;
    /* 给大盒子设置相对定位 */
    position: relative;
}

.grid_cell {
    width: 100px;
    height: 100px;
    border-radius: 6px;
    background-color: #ccc0b3;
    /* 给小盒子设置绝对定位 */
    position: absolute;
}

.number_cell {
    font-family: Arial;
    font-size: 30px;
    font-weight: bold;
    line-height: 100px;
    text-align: center;
    border-radius: 6px;
    position: absolute;
}